“Star Shots” (also called “spoke shots”) are used for quality assurance in radiation therapy systems. Such images are so named because they feature images of radiation beams that generally form a star or spoke pattern. Star shots are typically obtained on film placed parallel to a radiation beam (for a gantry star shot) or perpendicular to the beam (for couch, primary collimator, or MLC star shots). A star shot may be used to determine the radiation isocenter location (i.e., center of rotation) for components of a radiation delivery system, e.g., a gantry, couch, primary collimator, and MLC (multi-leaf collimator) relating to a radiation device such as a linear accelerator, a Cobalt-60 unit, a Radiation Therapy simulator, etc. Accordingly, a goal of the star shot is to ensure that the center of rotation of the element of the radiation delivery system (gantry, couch, primary collimator, or MIX) is within a certain specification during normal rotational operation, thereby ensuring that the beam of radiation hits its intended target when delivered from different angles.
Obtaining desired precision can be particularly challenging, especially in the case of a linear accelerator gantry weighing several tons, and where the typical requirement for a standard linear accelerator gantry is that the beams must intersect within a circle of two millimeters (mm) in diameter, and less than one mm for linear accelerators capable of performing stereotactic treatments. Compounding the difficulty is that, in addition to the size of the radiation beam intersection circle being within tolerance, it is also a requirement that the location of the beam intersection center be within 1 mm of the mechanical isocenter of the radiation delivery system for all mechanical motion in the system, including motion of the gantry, couch, primary collimator, and/or MLC collimator rotations.
Present systems for obtaining a star shot to determine radiation isocenters typically depend on use of film, which is difficult to use, expensive, and increasingly difficult to obtain. Further, present systems for obtaining a star shot do not provide information relating to the mechanical isocenter of a system unless the mechanical isocenter is manually labeled, which is inefficient and prone to error.